1. Field of the Invention
The present invention relates to a field emission device, and more particularly to a field emission device having a carbon nanotube (CNT) electron emitter source.
2. Prior Art
CNTs were originally produced as by-products of fullerene synthesis. CNTs have remarkable mechanical, electronic, and magnetic properties, which can be varied in principle by varying diameters and chiralities of the CNTs and the number of concentric shells that constitute each CNT. CNTs with extremely small diameters, hollow centers, chemical inertness, and mechanical strength have been used in a vast range of nanotube applications, such as electron field emitters, room-temperature transistors, and vehicles for hydrogen storage. In particular, there has been much effort to develop field emission displays (FEDs) using CNTs. CNT-FEDs have great potential for being applied to emissive devices including flat panel displays, cathode-ray tubes, backlights for liquid crystal displays, outdoor displays, and traffic signals.
Field emission is usually defined as “the emission of electrons from the surface of a condensed phase into another phase, frequently a vacuum, under the action of a high electrostatic field.” Since no thermal energy is necessary, field emission is called a “cold emission” process. A conventional FED depends on field emission from an array of small micro tips. A high electric field is applied between a gate electrode and tip emitters, whereby field emission of electrons from the tip emitters is induced. The ejected electrons strike a phosphor-coated screen, whereby visible light is emitted.
CNTs are generally considered to be the best field emitters, due to the very high electrical conductivity and extremely small tip area of each CNT. U.S. Pat. No. 6,642,639, granted to Won-bong Choi etc. on Nov. 4, 2003, discloses a field emission array adopting CNTs as an electron emitter source. Referring to FIG. 1, the field emission array includes a rear substrate assembly 100 and a front substrate assembly 200. In the rear substrate assembly 100, a plurality of cathodes 11 are formed as stripes over a rear substrate 10, and CNTs 11′ are deposited on the cathodes 11. The front substrate assembly 200 includes a combination of a front substrate 20 and a nonconductive plate 23. A plurality of anodes 21 are formed as stripes over the front substrate 20, and a phosphor layer 22 is deposited on each of the anodes 21. The nonconductive plate 23 has a plurality of gates 24 formed as stripes thereon, and this subassembly is combined with the front substrate 20 by spacers 25.
As shown in FIG. 1, the CNTs 11′ are densely arranged on the cathodes 11. Electrons can be emitted continuously at a high density from the CNTs 11′. However, tips of adjacent CNTs 11′ that are too close to each other may cause shielding of the applied electric field. If shielding exists, extra potential needs to be applied in order to overcoming the shielding. This goes against the demand for operation of field emission arrays at low voltages. In addition, such a CNT field emission array is complicated and difficult to produce and assemble. Furthermore, during production and assembly, impurities may become trapped in the CNT field emission array. This may shorten the useful operating lifetime of the CNT field emission array.